1. Field of the Invention
The present invention relates to system configurations and control methods for image projection apparatus implemented with a spatial light modulator (SLM). More particularly this invention relates to image projection system implemented with a spatial light modulator and adjustable varying light source to operate synchronously with the spatial light modulator to provide images with improved display qualities.
2. Description of the Related Art
Even though there have been significant advances made in recent years on the technologies of implementing electromechanical micromirror devices as spatial light modulator, there are still limitations and difficulties when these are employed to provide high quality image displays.
Specifically, when the display image is digitally controlled, the image qualities are adversely affected due to the fact that the image is not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices have drawn considerable interest because of their application as spatial light modulators (SLMs). A spatial light modulator requires an array of relatively large number of micromirror devices. In general, the number of devices required ranges from 60,000 to several million for each SLM. Referring to FIG. 1A for a digital video system 1, disclosed in U.S. Pat. No. 5,214,420, that includes a display screen 2. A light source 10 is used to generate an illumination light for the ultimate illumination for displaying images on the display screen 2. Light 9 generated is further concentrated and directed toward lens 12 by mirror 11. Lens 12, 13 and 14 form a beam columnator to columnate light 9 into a column of light 8. A spatial light modulator 15 is controlled by a computer through data transmitted over data cable 18 to selectively redirect a portion of the light from path 7 toward lens 5 to display on screen 2. As shown in FIG. 1B, the SLM 15 has a surface 16 that includes an array of switchable reflective elements, e.g., micromirror devices 32, such as elements 17, 27, 37, and 47 as reflective elements attached to a hinge 30. When element 17 is controlled in to operate along an ON-state position, a portion of the light from path 7 is redirected along path 6 to lens 5, where it is enlarged or spread along path 4 to impinge onto the display screen 2, so as to form an illuminated pixel 3. When element 17 is controlled to operate in an OFF-state position, the light is redirected away from the display screen 2 and hence the pixel 3 remains dark.
The on-and-off states of the micromirror control scheme, as that implemented in the U.S. Pat. No. 5,214,420 and by most of the conventional display systems, impose a limitation on the quality of the display. Specifically, in a conventional configuration of the control circuit, the gray scale (PWM between ON and OFF states) is limited by the LSB (least significant bit, or the least pulse width). Due to the on-off states implemented in the conventional systems, there is no way to provide a shorter pulse width than LSB. The least brightness, which determines gray scale, is the light reflected during the least pulse width. The limited gray scales lead to degradations of image display.
Specifically, FIG. 1C exemplifies a conventional circuit diagram of a control circuit for a micromirror, according to U.S. Pat. No. 5,285,407. The control circuit includes memory cell 32. Various transistors are referred to as “M*” where * designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5, and M7 are p-channel transistors; transistors M6, M8, and M9 are n-channel transistors. The capacitances, C1 and C2, represent the capacitive loads of the memory cell 32. Memory cell 32 includes an access switch transistor M9 and a latch 32a according to the basic static random access switch memory (SRAM) design. All access transistors M9 in a row receive a DATA signal from a different bit-line 31a. In order to access and write to a particular memory cell 32, a controller uses the row signal functioning as a word line to turn on an appropriate row select transistor M9. Latch 32a is formed from two cross-coupled inverters, M5/M6 and M7/M8, which permit two stable states. State 1 is Node A high and Node B low, and state 2 is Node A low and Node B high.
FIG. 1A shows a dual states switching with the control circuit controls the micromirrors to position either at an ON or an OFF orientation. The brightness, i.e., the gray scales of display for a digitally control image system, is determined by the length of time the micromirror stays at an ON position. The length of time a micromirror is controlled at an ON position is in turned controlled by a multiple bit word. For simplicity of illustration, FIG. 1D shows the “binary time intervals” when controlled by a four-bit word. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, 8 that in turn define the relative brightness for each of the four bits, where 1 is for the least significant bit and 8 is for the most significant bit. According to the control mechanism as shown, the minimum controllable difference between gray scales is a brightness represented by a “least significant bit” that maintains the micromirror at an ON position.
In a simple exemplary display system operated with an n bits brightness control signal for controlling the gray scales, the frame time is divided into 2n−1 equal time slices. For a 16.7 milliseconds frame period and n-bit intensity values, the time slice is 16.7/(2n−1) milliseconds
Having established these time slices for controlling the length of time for displaying each pixel in each frame, the pixel intensities are determined by the number of time slices represented by each bit. Specifically, a display of a black pixel is represented by 0 time slices. The intensity level represented by the LSB is 1 time slice, and maximum brightness is 2n−1 time slices. The number time slices that a micro mirror is controlled to operate at an On-state in a frame period determines a specifically quantified light intensity of each pixel corresponding to the micromirror reflecting a modulated light to that pixel. Thus, during a frame period, each pixel corresponding to a modulated micromirror controlled by a control word with a quantified value of more than 0 is operated at an on state for the number of time slices that correspond to the quantified value represented by the control word. The viewer's eye integrates the pixels' brightness so that the image appears the same as if it were generated with analog levels of light.
For addressing deformable mirror devices, a pulse width modulator (PWM) receives the data formatted into “bit-planes”. Each bit-plane corresponds to a bit weight of the intensity value. Thus, if each pixel's intensity is represented by an n-bit value, each frame of data has n bit-planes. Each bit-plane has a 0 or 1 value for each display element. In the example described in the preceding paragraphs, each bit-plane is separately loaded during a frame. The display elements are addressed according to their associated bit-plane values. For example, the bit-plane representing the LSBs of each pixel is displayed for 1 time slice.
Projection apparatuses such as described above generally use a light source such as a high-pressure mercury lamp such as a xenon lamp, or similar kinds of light sources. However, these types of light sources perform poorly in high-speed switching that alternates between the ON and OFF states. Accordingly, a light source is usually controlled to continuously operate in an ON state during the entire length of time when the projection apparatus is in operation. The light source that is continuously turned on generates a large amount of heat and wastes light and electricity.
There is also an increasing demand that projection apparatuses project images at a higher level of gray scale (gradation). Accordingly, a spatial light modulator has to be controlled to enable a projection apparatus to project images at a higher level of gray scale. However, the achievable improvement of the gray scale performance would be very limited if the improvements are to be achieved only through the control of the spatial light modulator. Some of the attempts to further improve the quality of image display have been disclosed in many patents, such as U.S. Pat. Nos. 5,214,420, 5,285,407, and published patent Applications. However, the disclosures including those included in the Information Disclosure Statement (IDS) have not provided effective solution to resolve the above discussed difficulties and limitations.